The present invention relates to a switching control circuit for use in a switching regulator of the self-excited ringing choke converter type.
A conventional switching regulator of the self-excited ringing choke converter type shown in FIG. 1 comprises a voltage transformer (for flyback choke) T1, a switching power transistor Q1, a switching control transistor Q2 for controlling on/off operation of the transistor Q1, a limiting resistor R1 for limiting the emitter current of the transistor Q1, a limiting resistor R2 for limiting the base current of the transistor Q1, a limiting resistor R3 for limiting the base current of the transistor Q2, a triggering resistor R4 for turning on the transistor Q1 when the regulator is powered, diodes CR1 and CR2, a capacitor C1 for turning on the transistor Q1 in the next cycle, and a smoothing capacitor C2.
When an astable DC power supply voltage (i.e., input voltage VIN) is applied to the switching regulator, the switching power transistor Q1 is turned on by means of the triggering resistor R4. The resistor R4 serves to turn on the transistor Q1 when the switching regulator is powered, so that the resistor R4 has a resistance of as high as several tens of kilohms to several hundreds of kilohms. When the transistor Q1 is turned on, a voltage substantially corresponding to the input voltage VIN is applied across a primary winding N1 of the voltage transformer T1. A voltage is induced in a base winding NB in proportion to the turn ratio of a base winding NB of the transformer T1 to the primary winding N1 thereof. The induced voltage causes a base current to flow through the base of the transistor Q1 through the diode CR1 and the resistor R2, so that the transistor Q1 is held in a stable ON state. In this case, the voltage polarities of the respective windings of the transformer T1 are given as shown in FIG. 3A. A collector current (primary current of the transformer T1) of the transistor Q1 increases linearly. In this case, a voltage induced in a secondary winding N2 of the transformer T1 is blocked by the output diode CR2, so that no current flows through a load L. When the collector current of the transistor Q1 increases in the manner described above and a ratio of the collector current to the base current of the transistor Q1 exceeds a current amplification or current transfer ratio h.sub.fe, a current flowing through the resistor R4 increases, so that a voltage drop across the resistor R4 is increased. The transistor Q1 is thus no longer saturated. A collector-emitter voltage VCE of the transistor Q1 abruptly increases. When this voltage increases, a voltage across the primary winding N1 of the transformer T1 decreases. Therefore, a voltage across the base winding NB decreases and the transistor Q1 is turned off.
When the transistor Q1 is turned off, energy which has been charged by the primary winding N1 of the transformer T1 is transferred to the secondary winding N2 and is discharged through the diode CR2. A current flowing through the diode CR2 is smoothed by the smoothing capacitor C2 and is transferred to the load L, thereby obtaining an output voltage VOUT. A current flowing through the diode CR2 has a peak value when the transistor Q1 is held OFF. As the transistor Q1 becomes gradually saturated, the current flowing through the diode CR2 decreases linearly. The peak value is given as (N2/N1).times.(maximum collector current immediately before the transistor Q1 is turned off), where N2/N1 is given in acccordance with the turn ratio of N1 (primary winding): N2 (secondary winding). The voltage polarities of the respective windings of the transformer T1 in the OFF state of the transistor Q1 are illustrated in FIG. 3B and are opposite to those shown in FIG. 3A. During the OFF state of the transistor Q1, the capacitor C1 is charged with the input voltage VIN through the resistor R4.
During the OFF state of the transistor Q1, a negative voltage is induced by the base winding NB of the transformer T1, as shown in FIG. 3B. This voltage is given as (V0+VFD0).times.(NB/N2), where V0 is the output voltage, VFD0 is the forward bias voltage of the output diode CR2, and NB/N2 is the turn ratio of the base winding NB to the secondary winding N2. As described above, the capacitor C1 is charged during the OFF state of the transistor Q1. When the charge current exceeds a sum of the voltage at the base winding NB of the transformer T1 and a base-emitter voltage VBE of the transistor Q1, the transistor Q1 is turned on. In the normal state, the transistor Q1 will not be turned on until energy at the secondary winding N2 of the transformer T1 is completely discharged. When the energy discharge is completed and the diode CR2 is turned off, no voltage is applied to the base winding NB of the transformer T1. In this case, when the charge current of the capacitor C1 exceeds the sum of the voltage (0 V) at the base winding NB and the voltage VBE of the transistor Q1, the transistor Q1 is turned on. As a result, a voltage substantially equal to the input voltage VIN is applied across the primary winding N1 of the transformer T1, as described above. A positive voltage is induced in the base winding NB in proportion to the turn ratio of the base winding NB to the primary winding N1 (see FIG. 3A). The induced voltage in the base winding NB causes the base current to flow through the transistor Q1 through the diode CR1 and the resistor R2. Therefore, the transistor Q1 is held in the stable conduction state. In this manner, the transistor Q1 is alternately turned on/off to generate a triangular wave current which is then smoothed by the capacitor C2, thereby obtaining the output voltage VOUT.
The transistor Q2 is arranged to stabilize the output voltage VOUT against variations in the input voltage VIN and load L. As shown in FIG. 1, and as is described in "Electronics/Dec. 21, 1978 pp. 100-104, "Flyback converters: solid-state solution to low-cost switching power supplies" by Robert J. Boschert", the transistor Q2 (also shown as transistor Q2 in FIG. 3 in this reference) is fed back with the output voltage. The base-emitter voltage of the transistor Q2 is controlled in accordance with the output voltage VOUT to change the OFF timing of the transistor Q1 so as to flow the base current of the transistor Q1 as the collector current of the transistor Q2. Therefore, the energy charge by the primary winding N1 of the transformer T1 is controlled, and hence the output voltage VOUT can be stabilized.
However, according to this conventional switching regulator, when the base current IQ1B of the transistor Q1 is withdrawn while the transistor Q2 is turned on to turn off the transistor Q1, a current IR1 flowing through the emitter current limiting resistor R1 is decreased by the base current component, as shown in FIG. 2E. Thus, in the conventional switching regulator, the ON state of the transistor Q2 becomes unstable, the storage time of the transistor Q1 increases, and a large power loss occurs. As a result, highly precise voltage control cannot be expected.